High-speed coherent transmission requires high standard coupling between fibers and photonic integrated chips (PICs). However most photonic integrated circuits operate at a specific polarization state, usually polarization of the fundamental mode. This is different from the situation in single mode fiber where radial symmetry guarantees the equivalence of different polarization orientations.
Inside a PIC, higher order modes will be excited as a consequence of imperfections such as side wall roughness, and as a consequence of reflections, transitions between multimode and single mode regions, and other interactions of the optical signal with the mechanical structure of the PIC.
Generally, integrated waveguides have two orthogonal transverse polarization states, TE and TM, each of which has a lowest ordered mode, TE0 and TM0, respectively. Between TE0 and TM0, the mode that has a lower mode number (i.e., the largest effective refractive index) is defined as the fundamental mode of the waveguide.
Higher order modes are generally easy to get rid of by tapering down the waveguide to a single mode region, as they become leaky modes. However, it is difficult to get rid of the TM0 mode because it is the lowest mode of the TM modes.
There are some typical and straightforward ways to eliminate a TM0 mode. One approach is to use a directional coupler to couple TM0 mode out of the waveguide. However this method is wavelength sensitive.
Another approach is to place metal at top of waveguide to absorb the TM0 mode. However, this approach is usually not compatible with standard CMOS processes and additionally introduces extra loss for the TE0 mode as well.
There is a need for systems and methods that maintain optical signals in well-defined polarization states.